1. Field of the Invention
The present invention relates to a thin film transistor having a semiconductor layer of an organic material or an inorganic material, and more specifically relates to an organic thin film transistor using an organic material, which can afford a thin layer transistor having high mobility and an excellent on-off property.
2. Description of the Related Art
Thin film transistors are widely used as a switching element for display in display devices such as liquid crystal display. Conventionally, thin film transistors (hereinafter also referred to as TFT) have been produced by using amorphous or polycrystalline silicon. CVD apparatuses for fabricating such TFT using silicon, however, are extremely expensive and thus fabrication of large-sized display devices using a TFT involved a problem of a remarkable increase in the fabrication cost. In addition, since the film forming process of amorphous or polycrystalline silicon is conducted at an extremely high temperature, there has been a problem that the kind of materials usable for a substrate has been limited and a lightweight resin substrate, for example, cannot be used.
In order to solve the above-mentioned problems, TFTs using an organic material instead of amorphous or polycrystalline silicon have been proposed. As a film forming method for forming a TFT by using an organic material, a vacuum deposition method and a coating method are known. According to these film forming methods, fabrication of large devices can be carried out while suppressing the cost increase, and the processing temperature required in the film forming process can be set to a relatively low temperature. Accordingly, TFTs using an organic material have an advantage that the limitation when choosing the material for the substrate is small, and thus practical application of such TFT is expected.
Recently, an increasing number of reports on TFTs using an organic material have been practically made (e.g., F. Ebisawa et al., Journal of Applied Physics, vol. 54, p 3255, 1983; X. Peng et al., Applied Physics Letter, vol. 57, p 2013, 1990; F. Garnier et al., Science, vol. 265, p 1684, 1994; K. Kudo, Thin Solid Films, vol. 331, p 51, 1998; etc.).
As an organic material used for an organic compound layer of a TFT, polymers such as a conjugated polymer and a thiophene (Japanese Patent Application Laid-Open No. 8-228034, Japanese Patent Application Laid-Open No. 8-228035, Japanese Patent Application Laid-Open No. 9-232589, Japanese Patent Application Laid-Open No. 10-125924, Japanese Patent Application Laid-Open No. 10-190001), a metal phthalocyanine compound (Japanese Patent Application Laid-Open No. 2000-174277) and a fused aromatic hydrocarbon such as pentacene (Japanese Patent Application Laid-Open No. 5-55568, Japanese Patent Application Laid-Open No. 2001-94107) are used alone or in a mixture with other compounds.
With the use of an organic material as a material for a semiconductor layer, it has become possible to use resins or plastics for a substrate of such devices, in addition to hard materials such as glass, which successfully led to flexibility of the whole device, and thus flexible organic TFTs are now being actively studied.
Further, since a coating process using a solution can be adopted as a process for producing an organic TFT, fabrication methods aimed at low cost, which employ a coating process or a printing process, are also being studied extensively.
FIG. 1 shows a cross-sectional structure of a typical organic TFT. The organic TFT-A has a gate electrode (layer) 14 and an insulator layer 16 on a substrate 11 in that order, and a source electrode 12 and a drain electrode 13 are formed on the insulator layer 16 at a predetermined interval of space. An organic semiconductor layer 15 is formed on the insulator layer 16 exposed between the electrodes 12 and 13 so as to include part of surfaces of the electrodes 12 and 13. In an organic TFT-A having such structure, the organic semiconductor layer 15 forms a channel region, and a voltage applied to the gate electrode 14 controls the electric current flowing between the source electrode 12 and the drain electrode 13 to operate on/off.
When the above-mentioned organic TFT-A is formed by coating and printing processes to fabricate a flexible device, formation of a substrate 11, an insulator layer 16 and an organic semiconductor layer 15 is important. In particular, the properties and characteristics of the insulator layer have a great influence on the performance of the organic TFT as a whole. If the insulator layer has poor resistance to electricity, leak current may be generated from the gate electrode, causing property deterioration of the organic TFT, which may result in breakdown of the element in worst cases. In addition, when forming an organic semiconductor layer by a coating process after forming an insulator layer by a coating process, the solvent dissolving the organic semiconductor material may dissolve the insulator layer, and thus fabrication of element is often impossible. Conventionally, since acrylic polymers (poly methylmethacrylate (PMMA), etc.) used as an insulator layer material dissolves relatively well in various solvents, it has been difficult to form an organic semiconductor layer by a coating process after forming a thin film of an insulator layer by a coating process. Thus, a dry process such as a vacuum deposition method is often used to fabricate an element. For such reason, development of an organic insulator layer material difficult to dissolve in a solvent is underway, but no satisfactory material has been found except for limited materials such as polyimide. Polyimide has an excellent insulating property and polyamic acid, a precursor thereof, has an excellent dissolution property. However, because a reaction from a thin film of polyamic acid, which is the precursor, to a thin film of polyimide must be conducted on a substrate, a process such as heating is required, and thus improvement is required in simplification of the process.